1. Technical Field
The present invention relates to a differential responsive hydraulic coupling arrangement for a 4WD vehicle, and more particularly to such a hydraulic coupling arrangement which takes advantage of a rotational difference between main or primary drive wheels (for example, rear wheels) of a vehicle and auxiliary or secondary drive wheels (for example, front wheels) to automatically switch a vehicle's drive condition from 2WD to 4WD.
2. Background Art
Generally a drive power from an engine of a vehicle is directly transmitted to main drive wheels to only drive the main drive wheels if the vehicle has just a 2WD mode. If the vehicle is a 4WD car, the vehicle is usually equipped with a coupling arrangement which has a following function and is particularly useful when running on a slippery road. When there is slippage or idle running between the main drive wheels and a snowy or slippery road which has a low friction coefficient, the drive power of the engine of the 4WD vehicle is also transmitted to auxiliary drive wheels. Since the drive power of the engine is transmitted to both the main and auxiliary drive wheels under a certain circumstance; this is the automatic switching of a drive mode from 2WD to 4WD. A typical example of this type of coupling arrangement is a differential responsive one which takes advantage of rotational difference between the primary and secondary drive wheels. One of the prevailing coupling arrangements of this differential responsive type is a viscous coupling arrangement which utilizes a high viscosity fluid.
The viscous coupling arrangement is easy to handle in a normal range (i.e., when there is an adequate rotational difference between main (rear) and auxiliary (front) drive wheels) since a torque transmitted to the auxiliary drive wheels rises gradually in such a range. However, the transmitted torque rises steeply in a range of very small rotational difference so that a torque more than necessary is frequently transmitted to the auxiliary drive wheels. It is caused, for example, even when the vehicle is running on an ordinary or general road of which surface has a certain size of concaves and convexes and a small rotational difference is generated between the front and rear drive wheels due to such small concaves and/or convexes. This results in a heat loss in a power transmission line for the auxiliary drive wheels since heat is unnecessarily generated upon unnecessary torque transmission to the auxiliary drive wheels and therefore this also results in a deteriorated fuel consumption rate. In addition, when the vehicle turns right or left, a "tight corner braking phenomenon" is significantly observed if the vehicle is provided with the viscous coupling arrangement. Furthermore, when a large rotational difference is created between the main and auxiliary drive wheels by a driver depressing an accelerator pedal, a large torque is transferred to the auxiliary drive wheels so that parts and components of a power transmission line should have particular strength and rigidity to cope with such high torque transmission.
Another example of the well known differential responsive coupling arrangements is a hydraulic coupling arrangement which is disclosed, for instance, in Japanese Patent Application, Publication No. 60-252026, the entire disclosure thereof is incorporated herein by reference. This hydraulic coupling arrangement uses the rotational difference between the main and auxiliary drive wheels to drive a hydraulic pump and engages or couples a clutch to connect the main and auxiliary drive wheels with each other thereby switching the drive condition from 2WD to 4WD. If there is no slippage between a road and the main drive wheels and there is no rotational difference between the main and auxiliary drive wheels, the hydraulic pump is not actuated and the clutch is not coupled so that the vehicle is operated in a 2WD condition. Therefore, the vehicle runs in a 4WD condition only when there is slippage between the road and the main drive wheels. In other words, the automatic drive mode switching from 2WD to 4WD only occurs when it is necessary.
As described in Japanese Patent Application Publication No. 60-252026, the hydraulic coupling arrangement has a torque transmission characteristic which is proportional to square of rotational difference. This characteristic is obtained by discharging through an orifice a part of an oil (or oil pressure) delivered from a hydraulic pump. This is especially effective in a range of very small rotational difference since it suppresses increase of the transmitted torque in this range. In other words, the hydraulic coupling arrangement does not have a drawback which the viscous coupling arrangement has. In addition, if a relief valve is provided in a hydraulic line, an upper limit or rise-up of the transmitted torque is determined or controlled, i.e., the relief valve functions as a torque limiter. Such a relief valve can prevent a high torque transmission to the auxiliary drive wheels so that the parts and components of a power transmission line related to the torque transmission to the auxiliary drive wheels may be reduced and/or simplified.
In the hydraulic coupling arrangement disclosed in the above-mentioned Japanese publication, however, the clutch is set to an engaged condition both when the main drive wheels are rotated at a higher speed than the auxiliary drive wheels and when the auxiliary drive wheels are rotated at a higher speed than the main drive wheels, so as to always switch to the 4WD mode upon rotational difference between the main and auxiliary drive wheels. This causes the following problem: when a vehicle turns right or left, the "tight corner braking phenomenon" occurs upon generation of rotational difference between the main and auxiliary drive wheels which is caused by different running routes of the front and rear drive wheels of the turning vehicle. The "tight corner braking phenomenon" prevents a smooth turning of the vehicle.
When the vehicle moves backward, the hydraulic pump is operated in a reverse direction. The coupling arrangement also brings the clutch in the engaged condition when the vehicle tries to make a right or left turn while it is moving backward. A smooth turning cannot be expected for the same reasons as mentioned above.
Other prior art arrangements are disclosed in, for example, Japanese Patent Application, Publication Nos. 1-250662 and 3-266726.